The Gifford-McMahon (G-M) type pulse tube refrigerator is a cryocooler, similar to G-M refrigerators, that derives cooling from the compression and expansion of gas. However, unlike the G-M systems, in which the gas expansion work is transferred out of the expansion space by a solid expansion piston or displacer, pulse tube refrigerators have no moving parts in their cold end, but rather an oscillating gas column within the pulse tube that functions as a compressible displacer. The elimination of moving parts in the cold end of pulse tube refrigerators allows a significant reduction of vibration, as well as greater reliability and lifetime, and is thus potentially very useful in cooling cryopumps, which are often used to purge gases from semiconductor fabrication vacuum chambers.
G-M type pulse tube refrigerators are characterized by having a compressor that is connected to a remote expander by high and low pressure gas lines. The pulse tube expander has a valve mechanism that alternately pressurizes and depressurizes the regenerators and pulse tubes to produce refrigeration at cryogenic temperatures.
Two stage G-M refrigerators, which are presently being used to cool cryopumps, cool a first stage cryopanel at about 60 K and a second stage cryopanel at about 15 K. The expander is usually configured as a stepped cylinder with a valve assembly at the first stage warm end, a first stage cold station (60 K) at the transition from the larger diameter first stage to the smaller diameter second stage, and a second stage cold station (15 K) at the far end. The cryopanels are typically axi-symetric around the cold finger. The cryopump operates equally well in all orientations.
Longsworth, U.S. Pat. No. 4,150,549, dated Apr. 24, 1979 and entitled “Cryopumping Method and Apparatus”, describes a typical cryopump that uses a two stage G-M refrigerator to cool two axi-symetric cryopanels. The first stage cools an inlet (warm) panel that pumps group I gases, e.g. H2O, and blocks a significant amount of radiation from reaching the second stage (cold) panel but allows group II, e.g. N2, and m, e.g. H2, gases to pass through it. The Group II gases freeze on the front side of the cold panel(s) and Group III gases are adsorbed in an adsorbent on the backside of the cold panel(s).
Unlike a typical GM expander that has a single stepped cylinder that lends itself to attaching axi-symetric cryopanels, the two stage pulse tube expander has two pulse tubes and two or more tubes to house the regenerators. The pulse tubes themselves tend to be as long as the most common size cryopump which has a diameter of 200 mm. G-M type pulse tube refrigerators that operate below 20 K have the disadvantage of requiring that the hot end of the pulse tube be above the cold end in order to avoid the thermal losses associated with convective circulation within the pulse tube. Conventional two-stage GM type pulse tube refrigerators typically have the valve mechanism and the hot end of the pulse tube on top. This enables the heat that is rejected at the hot end of the pulse tube to be easily transferred to the low-pressure gas and returned to the compressor where it is rejected.
Most cryopumps are mounted below the vacuum chamber where space above the cryopump housing is very limited. Having the valve mechanism above the cryopump housing limits the applications of the cryopump. Thus, any options to orient the pulse tube refrigerator with the valve behind or below a cryopump housing that has a side inlet are highly desirable. The first and second stage pulse tubes are two separate tubes that have two or three regenerator tubes with them. The arrangement of the pulse tubes and regenerators in the cryopump housing makes it very difficult to make conventional axi-symetric cryopanels because they have so many cut outs to fit around the tubes. This problem has not been recognized or solved in the prior art.
It is an object of the present invention to provide an arrangement of the tubes within the cryopump housing that facilitates the fabrication and installation of the cryopanels.